How can use the mapping node for two noise textures
You can use a geometry node which allows to use the rendered position to do this kind of thing.



The additional nodes can be plugged before the top left colorramp of your material:



From left to right (the top nodes line):


Rendered position: the position is given from the object origin
Separate XYZ, in order to keep only Z (for the poles)
Absolute: the sphere here is 2 meters height and its origin is a the middle, so "absolute" gives a value between 0 and 1
A color ramp to be tuned so that the earth surface is more or less far from the poles
Multiply: to mix your noise input with the previous ramp which translates the Z value
The last output is injected in the color ramp of your initial material (or at least I've tried to mimic it)